3.1 Effect of different substrates on plant growth
Except plant height, no significant difference in the growth status of V. natans was observed among 4 different substrate groups (P > 0.05) (Fig.1a). The mean plant height in the VE group was significantly higher than that in other groups (CK and 3 other treatment groups) during the whole experimental period (P < 0.05) (Fig.1a). After 40-day culture, VE group reached the maximum plant height (30.5 cm). After 20-day culture, VR group reached the maximum number of leaves (16). In addition, the number of leaves in all 4 treatment groups was higher than that in CK group during the culture period (Fig.1b), but difference was not significant (P > 0.05). During the growth of V. natans under the four substrate treatments, the change trend of root length was consistent with that of leaf number (P > 0.05) (Fig.1c). During the culture period, no significant difference in the biomass of V. natans was observed among all the groups (treatment groups and CK) (P > 0.05) (Fig.1d). After 40-day culture, the biomass in CE and VE groups reached the maximum, which was 3.81 g and 6.29 g, respectively.
3.2 Effect of different substrates onphysiological properties of plants
In ME group, the contents of Chl-a and Chl-b showed the same change trend, which increased first and then decreased (Fig.2a and Fig.2b). On the 20th day post culture, the contents of Chl-a and Chl-b in the ME group reached the maximum, which were 2.01 mg g-1 and 1.37 mg g-1, respectively. On the 40th day post culture, the total chlorophyll content in all the groups except group VE (which was slightly decreased) was increased, compared with the initial value (on day 10), and the highest total chlorophyll content was observed in ME group (2.4mg g-1) (Fig.2c). During the whole experiment, the average total chlorophyll content in V. natans leaves in CE, ME, VE, VR, and CK groups was 0.99 mg g-1, 2.40 mg g-1, 1.10 mg g-1, 1.03 mg g-1, and 0.99 mg g-1, respectively.
From the early to middle stage of culture (day 10-20), CAT activity was increased in all groups, and the highest CAT activity was observed in VR group (489.83 U g-1 min) (Fig.3a). However, on the 40th day post culture, CAT activity in all 4 treatment groups showed a downward trend, which was lower than that in CK group. No significant difference in the SOD activity in V. natans leaves was observed among CE, ME, or VR groups during the whole culture period (P > 0.05) (Fig.3b), and on day 30, the SOD activity in CE, ME, and VR groups reached the maximum, which was 331.18 U g-1 min, 458.50 U g-1 min, and 215.72 U g-1 min, respectively. On day 40, SOD activity in V. natans leaves in all the treatment groups reached the lowest value, which was similar to the change trend of CAT activity.
The MDA content in V. natans leaves was significantly different among all 5 groups (4 treatment groups and CK) during the culture period (P < 0.05) (Fig.3c), and the MDA content in CE and ME groups was higher than that in VE and VR groups during all the periods. Compared with the CK group, the 4 treatment groups reached the maximum MDA content earlier. On day 10, the MDA content in V. natansleaves in CE, ME, VE, and VR groups reached the maximum, which was 53.73 nmol g-1, 61.38 nmol g-1, 41.37 nmol g-1, and 41.08 nmol g-1, respectively.
The root activity of V. natans in all 5 groups increased first and then decreased (Fig.3d), and it reached the highest from 20 to 30 days post culture. The root activity of V. natans in the four treatment groups was higher than that in the CK group throughout the entire period, but without statistical significance (P > 0.05). During the whole experiment period, the mean root activity ofV. natans in CE, ME, VE, VR, and CK groups was 67.14 U g-1, 34.78 U g-1, 74.58 U g-1, 85.28 U g-1, and 28.48 U g-1, respectively.